The present invention relates generally to ultracapacitors with polymer active materials, and, more particularly, to a method for fabricating polymer ultracapacitors.
Electrochemical capacitors, also called supercapacitors or ultracapacitors, are energy storage devices which can store more energy than traditional capacitors and discharge this energy at higher rates than rechargeable batteries. In addition, the cycle life of electrochemical capacitors should far exceed that of a battery system. Ultracapacitors are attractive for potential applications in emerging technology areas that require electric power in the form of pulses. Examples of such applications include digital communication devices that require power pulses in the millisecond range, and traction power systems in an electric vehicle where the high power demand can last for seconds up to minutes.
Battery performance and cycle life deteriorate severely with increasing power demand. A capacitor-battery combination has been proposed where the capacitor handles the peak power and the battery provides the sustained load between pulses. Such a hybrid power system can apparently improve the overall power performance and extend battery cycle life without increase in size or weight of the system.
An ultracapacitor is basically the same as a battery in terms of general design, the difference being that the nature of charge storage in the electrode active material is capacitive, i.e., the charge and discharge processes involve only the movement of electronic charge through the solid electronic phase and ionic movement through the solution phase. Energy densities of ultracapacitors are much higher than those of conventional capacitors, but typically lower than those of advanced batteries. However, compared to batteries, higher power densities and longer cycle life have been either demonstrated or projected. These latter advantages of ultracapacitors over batteries are achievable because no rate-determining and life-limiting phase transformations take place at the electrode/electrolyte interface.
The dominant ultracapacitor technology has been based on double-layer type charging at high surface area carbon electrodes, where a capacitor is formed at the carbon/electrolyte interface by electronic charging of the carbon surface with counter-ions in the solution phase migrating to the carbon in order to counterbalance that charge.
Conducting polymers have been investigated for use in ultracapacitors. Higher energy densities can be achieved because charging occurs through the volume of the active polymer material rather than just at the outer surface. When a conducting polymer is being p-doped (positively charged), electrons leave the polymer backbone generating an excess of positive charge; anions from the electrolyte solution migrate into the polymer matrices to counter the positive charge. In the case of n-doping of conducting polymers, the polymer backbone becomes negatively charged by the addition of electrons from the external circuit; cations enter the polymer matrices from solution to balance the negative charge.
An object of the present invention is to provide an ultracapacitor that is compact and lightweight. The present invention also provides methods of electrode fabrication, cell assembly and packaging.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a sealed ultracapacitor assembly. First and second electrodes are formed of first and second conducting polymers electrodeposited on porous carbon paper substrates, where the first and second electrodes each define first and second exterior surfaces and first and second opposing surfaces. First and second current collector plates are bonded to the first and second exterior surfaces, respectively. A porous membrane separates the first and second opposing surfaces, with a liquid electrolyte impregnating the porous membrane. A gasket formed of a thermoplastic material surrounds the first and second electrodes and seals between the first and second current collector plates for containing the liquid electrolyte.
In another characterization of the invention, a method is presented for forming a sealed ultracapacitor. First and second carbon disk electrodes are bonded on first and second current collector plates, respectively, and conducting polymer active material is electrodeposited onto the first and second carbon paper disk electrodes to produce first and second capacitor electrodes. First and second gasket seal rings are bonded to the first and second current collectors. A porous separator is sealed to the first gasket seal ring to cover the polymer active material on the first capacitor electrode. A first electrolyte fill tube is inserted between the separator and the first gasket seal ring on the first capacitor electrode. The second gasket seal ring is aligned facing the first gasket seal ring and a second electrolyte fill tube is inserted between the first and second gasket seal rings. The first and second gasket seal rings are then bonded together. A liquid is introduced through the first electrolyte fill tube while drawing a vacuum on the second electrolyte fill tube to impregnate the porous separator with the electrolyte. The first and second electrolyte fill tubes are then sealed to form the sealed ultracapacitor.